Recently, instead of a fluorescent lamp or bulb having a filament, a semiconductor light emitting element such as an LED with a long life and low power consumption is used as a light source of an illumination apparatus.
FIG. 3 illustrates a schematic configuration of a conventional illumination apparatus B1, which includes a power supply circuit 101, a light source unit 102 and an apparatus housing 104.
The power supply circuit 101 consists of a non-isolated AC/DC converter to output a DC power obtained by rectifying (full-wave rectifying or half-wave rectifying) an AC voltage Vi2 inputted from an AC power source 110 and stepping up the rectified voltage.
The light source unit 102 includes a substrate (not shown) mounted with semiconductor light emitting elements 103 connected, e.g., in series, and a DC voltage Vo2 outputted from the power supply circuit 101 is applied between both ends of the series circuit of the semiconductor light emitting elements 103. That is, the semiconductor light emitting elements 103 are turned on by the DC power supplied from the power supply circuit 101.
In addition, an AC power is supplied to the input of the power supply circuit 101 from the AC power source 110 through a pair of power feed lines Wb1 and Wb2, and a switch SW100 is located in the power feed line Wb2. The switch SW100 constitutes a single pole switch of the AC power source such as a wall switch for home, and by turning on/off the switch SW100, it is possible to allow or block the power supply from the AC power source 110 to the power supply circuit 101 to switch on/off the semiconductor light emitting elements 103.
Further, the apparatus housing 104 is formed of a conductive material such as metal, and the power supply circuit 101 and the light source unit 102 are mounted on the apparatus housing 104. In FIG. 3, only a part of the apparatus housing 104 is illustrated.
In addition, the AC power source 110 has a ground phase, and the ground phase (on the side of the power feed line Wb2 in which the switch SW100 is located) is grounded through a ground path Wb3. Further, the apparatus housing 104 of the illumination apparatus B1 is also grounded through a ground path Wb4. That is, the apparatus housing 104 has the same potential as the ground phase of the AC power source 110.
However, in the conventional illumination apparatus B1, even in a state where the switch SW100 is turned off, the semiconductor light emitting elements 103 emit light slightly to generate slight light emission and there was a problem such that the semiconductor light emitting elements 103 seem to be turned on.
This is due to a stray capacitance Cb generated between the light source unit 102 and the apparatus housing 104 when the light source unit 102 is attached in the vicinity of the apparatus housing 104. Specifically, even in a state where the switch SW100 is turned off, there exists a current loop of the AC power source 110—the power feed line Wb1—the power supply circuit 101—the semiconductor light emitting elements 103—the stray capacitance Cb—the apparatus housing 104—the ground path Wb4—the ground path Wb3—the AC power source 110. The current from the AC power source 110 flows into the current loop, and the semiconductor light emitting elements 103 emit light slightly by this current loop even in a state where the switch SW100 is turned off. In addition, the stray capacitance Cb is formed by a stray capacitance between the semiconductor light emitting elements 103 and the apparatus housing 104, a stray capacitance between the substrate on which the semiconductor light emitting elements 103 are mounted and the apparatus housing 104, a stray capacitance between a case of the light source unit 102 and the apparatus housing 104 and the like.
In order to solve this problem, in the conventional illumination apparatus B1, as shown in FIG. 3, in the light source unit 102, capacitors C100 are connected with the semiconductor light emitting elements 103 in parallel. This configuration is intended, by using the capacitors C100, to limit the stray capacitance Cb generated between the light source unit 102 and the apparatus housing 104 and to block the current loop including the semiconductor light emitting elements 103, thereby preventing slight light emission.
However, in case of using the capacitors C100, at least the capacitors C100 of the same number as the semiconductor light emitting elements 103 are necessary, and the illumination apparatus B1 includes a large number of components. Accordingly, it becomes a factor of inhibiting miniaturization and cost reduction.
In addition, if the switch SW100 is located in the power feed line Wb1 that is not grounded among the power feed lines Wb1 and Wb2, since the current loop is not formed, it is possible to prevent slight light emission. However, the switch SW100 needs to be wired considering a ground situation and it was difficult to obtain a good workability.